The present invention relates to a computer with a read only memory whose contents can be partially changeable.
In computers, especially microcomputers, read only memories (ROMs) are used for saving fixed data and programs. ROMs fall into several categories, the most common three being:
(1) Mask programmed ROMs, usually referred to simply as ROMs, in which the information is inserted during manufacturing and cannot be changed. Mask programmed ROMs have the advantage that they are comparatively cheap and reliable.
(2) PROMs in which the information is inserted after manufacturing but cannot be changed, either. They are reliable and not too expensive.
(3) EPROMs in which the information is inserted after manufacturing and can be changed. However, they are expensive and less reliable.
In view of the above, mask ROMs are widely used in devices which require high reliability and low cost. A typical example of such devices is an electronic control system for automobiles. Accordingly, if a change of the information stored in a mask ROM is required, the mask ROM is replaced in whole by a new one. However, it is costly and time consuming to prepare a new ROM pattern. Besides, all the information must be replaced even if only a portion, one bit or two, for example, requires a change.
There has been proposed a method for modifying a portion of the information of ROM in Japanese patent provisional publication Sho. 54-160141, for example. However, this method is not suitable for a system such as control systems for automobiles, because this method requires a complicated and costly storage means.